The human form of mad cow disease is much more common than first thought, scientists in the United Kingdom have found.

Research published in the British Medical Journal shows almost 32,000 people in Britain are carrying the variant protein - double the most recent estimate.

The disease created widespread concern in the 1980s and '90s when pictures of distressed and sick cattle were shown in the media.

Some humans who consumed meat from the diseased cattle then began to develop a related form of mad cow disease - a crippling degenerative brain disease known as Creutzfeldt-Jakob disease (CJD).

The latest findings are expected to again throw the spotlight on the millions of people exposed to the disease, including many Australians who were living in the UK at the time.

Professor Colin Masters, a neurologist at the Institute of Neuroscience and Mental Health at the University of Melbourne, says the research marks the first time the possible number of people directly affected by CJD has been examined.

"So far there's been 177 people who've actually developed full-blown disease of this so-called variant, Creutzfeldt-Jakob disease, but that was really just the tip of the iceberg," he told PM.

"We always suspected that there'd be a very large number of individuals who were exposed, have developed the infection, but for various reasons have never developed the full-blown disease."

The big question remains unanswered: just how many of the millions exposed to mad cow disease are actually likely to develop full-blown CJD symptoms?

"In certain individuals, in most individuals, it probably will never get to the brain and remains in the gut," Professor Masters said.

"And this is what these current figures are showing: that one in 2,000 individuals in the UK potentially have this infectious protein in their peripheral circulation or in the gut tissues."

People unable to give blood in Australia

Professor Masters believes that many more people will die from other diseases than from CJD, because it is a slow burner and many people carry immunity to its effects.

He would like more focus to be placed on finding a reliable test for the disease in humans.

The lack of a test has led to caution from groups like the Red Cross Blood Service in Australia.

"This is why we've had rules in place that have not allowed people who might have been exposed to mad cow to donate blood," spokeswoman Kathy Bowlen said.

"It's been a difficult position for the Blood Service to take and a lot of people who've spent time in the UK and that's meant they've not been able to donate blood in Australia have been puzzled by it and some are unhappy about it.

"Essentially for us, the position has been: if you might have been exposed to mad cow, can we eliminate the risk of passing that onto patients who need your blood?"

Anyone who lived in the UK from 1983 until 1996 for a period of six months or more is excluded from giving blood in Australia.

The Blood Service also excludes blood donations from people exposed to malaria and other diseases, people with hepatitis and with HIV, IV drug users and from people who engage in unsafe sexual practices.